1. Field of the Invention
This invention relates to calcination of preheated, pulverous, raw material, such as cement raw meal, consisting of or containing lime. The invention relates to an improved method of at least partially calcining such pulverous, raw material and improved calcination plants for treating such raw materials according to the improved method wherein heat is supplied before the material is subjected to any finishing calcination and/or other heat treatment, if any.
2. Description of the Prior Art
Calcination of pulverous raw materials such as cement raw meal is to be understood as an expulsion of carbon dioxide from calcium carbonate by an endothermic process (i.e. a process in which heat is absorbed) according to the equation: EQU CaCO.sub.3 .fwdarw. CaO + CO.sub.2
when the raw material is cement raw meal, a finishing heat treatment following the calcination is a sintering by which cement clinker is produced. Sintering is an exothermic process characterized by, or formed with, evolution of heat.
The heat necessary for carrying through the conversion of the cement raw meal to cement clinker is usually provided by burning fuel, which together with combustion air, is introduced into a combustion chamber and forms smoke gas. As a result, the energy contained in the fuel is released for heating the smoke gas to a high temperature. The hot smoke gas is then brought into contact with the raw meal to be heat treated. The heat is mainly used for preheating and calcining the raw meal, its sintering being as mentioned an exothermic process; in practice, however, heat must be supplied in order to start the sintering.
Owing, among other things, to the presence of alkalis in the raw meal and the consequent drawbacks it is sometimes preferred to carry through the preheating and calcining of the raw meal by hot gas from one source of heat and the initiation of its sintering by hot gas from another source of heat.
In the case of calcination of cement raw meal it is desirable to carry through this process at a low temperature. However, it is difficult to do that by means of smoke gases having a high temperature since there is then a great risk of excessive heating of the raw meal occurring locally and temporarily. Even excessive heating of a part of the raw meal for a short time may involve expulsion of alkali vapors or the production of melts which may give rise to cakings. Excessive heating of the raw meal at the calcination stage may also prevent chemical reactions intended to take place at a later stage of the whole process for the manufacture of cement clinker. For example, a clinker mineral formation at the stage of the total process at which the calcination is to take place will involve a disadvantageous development of the whole process.
U.S. Pat. No. 3,203,681 to Rosa, et al, relates to a process wherein heat for carrying through the calcination of preheated cement raw meal derives from hot gases having a temperature higher than the calcination temperature. The gases are produced in a separate chamber and are passed upwardly in a riser column in which the raw material is suspended and entrained by the gases thus produced. U.S. Pat. No. 3,452,968 to Shimizu, et al. relates to a process for roasting fine ore wherein preheated raw meal and fuel are ejected individually into a rotating flow of gas ascending upwardly in a calcining chamber. Combustion and the roasting reaction are thus caused in the violently diffusing turbulent flow. Neither of these prior art patents disclose or suggest a method of heat treating a pulverous, raw material or a plant for practicing the method such as I have invented. According to my invention, a raw material consisting entirely of, or at least containing a portion of, lime is at least partially calcined substantially isothermically (i.e. constant temperature conditions) at relatively low temperatures while substantially eliminating the disadvantages of the presently known systems.